In commercial practice, as for example in the chlor-alkali industry, diaphragm type electrolytic cells have been most common until recent years. In these diaphragm type cells the diaphragms separated the anolyte and catholyte chambers. Normally, these diaphragms were simply an asbestos mat or the like and in any case were hydraulically permeable. Normal practice was to build such an asbestos diaphragm between opposed anodes and cathodes so that the diaphragm extended therebetween to the walls of the cell or to clamping means at the wall surfaces. Another arrangement common in the prior art was to have the diaphragm material to entirely surround the cathode and in fact be deposited on the cathode so as to cover all surfaces thereof. In all cases however, the anolyte, due to the hydraulic permeability of the diaphragm would be essentially in contact with all surfaces of the cathode whether they be active or inactive.
This same practice has been carried over into electrolytic cells employing hydraulically impermeable membranes between anolyte and catholyte chambers. Standard practice has been to place a membrane wall between anode and cathode to eliminate hydraulic communication between each chamber. Of necessity, such use of a sheet membrane covered both active and inactive areas between the electrodes.
Furthermore, in membrane cells wherein a single cathode forms a cell with two anodes the practice has been to enclose the cathode in a hydraulically impermeable membrane envelope wherein the membrane prevented liquid communication with the cathode in the electrolytic cell. Such an envelope was open only at the top above the liquid level to allow the escape of gases formed during electrolysis. Here again, all areas of the cathode were covered without discrimination between active and inactive surfaces. Such complete enclosure of the cathode with a hydraulically impermeable membrane is detrimental for a number of reasons. For example, consider such a membrane completely enclosing the cathode in a chlor-alkali cell. Between the inactive regions of the cathode and membrane there will be caustic. However, since there will be little electrolysis occurring in these areas, a significant amount of back-migration of hydroxyl ion would occur which would result in a serious loss in cell current efficiency.
Also, the membrane material is more expensive compared to a nonpermeable material such as used in the instant invention to separate the anolyte and catholyte chamber in inactive electrode areas.
Another advantage of the instant invention over the prior are practices is that fabrication of a membrane envelope is extremely difficult and in any event more difficult than fabricating an envelope of nonpermeable material with membrane windows. Due to its relatively delicate nature, membranes are more difficult to fabricate especially in cases where you are attempting to form an envelope which would completely surround an electrode. The potential leakage problems would exist wherever the membrane was joined to itself and especially in areas of stress such as along edges when the membrane is shaped at great angles to closely fit the cathode shape.
The advantages of the instant invention are further magnified when it is considered that most commercial electrolytic operations consist of a large plurality of individual electrolytic cells wherein the use of impermeable envelopes having hydraulically permeable membrane windows therein between active areas of the electrodes are used. Such envelopes can be interconnected and preferably are as for example, when a multi-cell diaphragm type operation is converted to a membrane type unit.